1. Field of the Invention
This invention relates to a color image forming apparatus adapted to form a toner image on a belt-shaped image retainer by an electrophotographic system, to transfer the toner image on a transfer material and to obtain an object image.
2. Description of the Prior Art
Many methods and apparatuses for obtaining color images by using electrophotography have been proposed. As disclosed in, for example, Japanese Patent Laid-open No. 100770/1986, there is a method of obtaining a color copy by forming a latent image on a photosensitive drum serving as an image retainer in accordance with a color separation number of a document image, developing the latent image on the drum, transferring the resultant image onto a transfer drum every time the development of the latent image is completed to form a multi-color image on the transfer drum, and thereafter transferring the multi-color image onto a transfer material to obtain an object color copy. An apparatus realizing this method needs to be provided with a large transfer drum where a one-frame image is transferred on the circumferential surface in addition to a photosensitive drum. Consequently, it is unavoidable that this apparatus has a large and complicated structure.
There is another method disclosed in, for example, Japanese Patent Laid-open No. 149972/1986, which comprises the steps of forming a latent image on a photosensitive drum in accordance with the color separation number of a document image, developing the latent image on the drum, and transferring the resultant image onto a transfer material every time the development of the latent image is completed, to obtain a multi-color copy. In this method, it is difficult to accurately superpose each of monochromatic images upon one another, so that a high-quality multi-color copy cannot be obtained.
There is also another method of obtaining a color image, which comprises the steps of repeating formation of a latent image on a photosensitive drum in accordance with the color separation number of a document image and development of the latent image with color toners, superposing color toner images upon one another on the photosensitive drum, and then transferring the resultant image onto a transfer material. The basic processes for formation of such a multi-color image are disclosed, for example, in Japanese Patent Laid-open Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985 and 158475/1985.
In a multi-color image forming apparatus for obtaining a color image by such an image superposing step, a plurality of developing devices storing different color toners therein are arranged around a photosensitive drum, and a latent image on this drum is developed by rotating the same drum generally a plurality of times to obtain a color image.
In addition to the photosensitive drum which has a photoconductive material applied or evaporated on the circumferential surface of the drum as described above, a belt-shaped image retainer which has a photoconductive material applied or attached to a flexible belt has also been proposed.
In Japanese Patent Laid-open No. 129678/1986, for example, there is proposed a monochromatic laser printer utilizing a belt-shaped image retainer. In this printer, the shape of the belt-shaped image retainer is determined by training the image retainer around rollers including a drive roller. Thus, the printer is advantageous in that it can be constructed in a compact form by utilizing the space efficiently. Further, the printer is of a clamshell structure with the belt-shaped image retainer being constructed as a unit adapted to be inserted removably in the printer body. Accordingly, it is easy to correct jamming or to replace parts. In the printer of the prior art, however, no measures are adopted to protect the image retainer against damage which would be caused when the clamshell is opened or when the unit is inserted or removed.
Further, since the belt-shaped image retainer can be moved along a line of small curvature, a roller with a small diameter can be used so that imperfect separation of transfer material is prevented by utilizing the part with the curvature for separating the transfer material.
Some of the image forming apparatuses are adapted such that the inside of the apparatus is opened or a process unit of cartridge type is removed from the apparatus body for the purpose of correcting jamming and replacing or cleaning components. If the apparatus is opened or the components are removed in this manner, the exposed components are apt to be damaged or contaminated with dust. For solving such, it has also been proposed to provide a protective cover. For example, Japanese Patent Laid-Open No. 34546/1984 proposes a protective cover for protecting an exposure section, Japanese Patent Laid-open No. 61865/1984 proposes a protective cover for protecting a photosensitive body of a transfer section and Japanese Patent Laid-open No. 61860/1984 proposes a protective cover for protecting a developing section.
Further, in the copier industry, there is an increasing demand for color images in place of monochromatic images because the color images can carry more data than the monochromatic ones.
In order to meet such demand, a color copying machine utilizing the belt-shaped image retainer as described above, the Sharp Full Color Copying Machine CX-7500, for example, has been developed and is available on the market. This color copying machine is of analogue mode and comprises a machine body including a paper feeding system and an upper casing adapted to be openable with respect to the machine body, a belt-shaped image retainer, a plurality of developing devices juxtaposed over the image retainer and an electric control system. The color copying machine is adapted to form toner images of respective colors sequentially on the belt-shaped image retainer by rotating the image retainer several times, while each toner image formed on the image retainer is transferred to a transfer belt. As a result, the toner images of the respective colors are superposed on the transfer belt to provide a color toner image. This color toner image is transferred onto a transfer paper and fixed to form a color image.
However, in the conventional color copying machine as described above, the upper casing which is adapted to be openable with respect to the machine body contains many components including the belt-shaped image retainer, several developing devices, analogue control system etc., and it is so heavy that it is difficult to open. In addition, since no measures are adopted for protecting the surface of the image retainer exposed to the outside when the upper casing is opened, the copying machine encounters problems because the image retainer is apt to be subjected to fatigue deterioration due to light and it is apt to be damaged or contaminated with dust.
Further, since the conventional copying machine employs an image forming method in which the toner images of the respective colors formed on the image retainer by several rotations of the image retainer are superposed on the intermediate transfer belt, accuracy of superposition is very low and the color image can not be obtained with high resolution.
In order to solve the problem of protection of the exposed surface of the image retainer, various measures have been proposed by Japanese Patent Laid-open No. 34546/1984, No. 61865/1984 and No. 61860/1984, for example. All this prior art, however, relates to a reproducing machine adapted to form monochromatic images in an analogue mode by utilizing an image forming drum.
Methods of forming color images have been proposed in Japanese Patent Laid open Nos. 75850/1985 and 76766/1985, for example, wherein toner images of respective colors are superposed on an image retainer to form color toner images and the color toner images are transferred collectively onto a transfer paper and fixed thereon to form a color image. According to such method, the color image can be obtained without color misregistration and with high resolution since the toner images of the respective colors are superposed directly on the image retainer. However, both of these prior art apparatuses utilize an image forming drum for image formation and do not disclose any measures for protecting the surface of the image forming drum.
Further, in the color image forming apparatus utilizing the belt-shaped image retainer as described above, the image forming means including charging means, image exposure means and a plurality of developing devices are juxtaposed mainly on the lower part of the outer periphery of said belt-shaped image retainer. Accordingly, if the belt-shaped image retainer takes the form of a removable cartridge, a wide range of a photosensitive material opposite to the image forming means will be exposed and the risk of damaging the image retainer becomes very high.